1. Field of Invention
The present invention relates to welding and more particularly to laser welding.
2. Description of the Prior Art
Metal welding techniques have been investigated and practiced over a considerable period of time, and various refinements have been developed during this period. In a conventional arc welding process which is typical of the metal welding processes in use, various impurities tend to contaminate the weld zone during the welding process and degrade the quality of the weld. A principal source of these impurities is the absorbed gas which is found on the surface of the base metal being joined, and on the filler metal used in the joining process. Another source of impurity is the solid matter often present on the surface of the base metal. In addition to the sources associated with the metal to be joined, various impurities including gases and dust are present in the atmosphere surrounding the weld region and find their way into the weld zone. A further source of weld contamination is the impurities present in the metal itself; typically these impurities consist of oxide and sulphide inclusions which occur in the metal during initial manufacture. A still further source of contamination is the impurities which are often found in the shield gas, although additives are sometimes purposely added to the shield gas in order to stabilize the arc.
Each of these sources of contaminants has a tendency to appear in the weld region of the workpiece in an arc welding process due to the dynamics involved. In a typical arc welding process, the arc assumes a dome profile over the work zone, providing a substantially complete cover over the molten pool which is formed during the welding process, with a portion of the pool extending from the trailing edge of the dome. A basic premise which is generally recognized is that fluids tend to move from high energy regions such as the arc formed in a welding process to regions of lower energy. In the interaction zone formed between the arc and the workpiece during the welding process, the arc is the higher energy region and therefore fluids tend to move from the arc toward the workpiece. When shielding gases are used in such a process, they are generally directed along the axis of the filler rod which causes them to be directed essentially toward the workpiece. In addition, the gases in the arc are accelerated toward the workpiece because of the general tendency to move from the higher energy region toward a region of lower energy. These two factors combine to give the gas a momentum which tends to cause the weld pool, which is predominately within the arc zone, to pick up some of the contaminants that have been vaporized and driven toward the workpiece. In addition, contaminants within the material are driven back into the weld zone by both the energy gradient and momentum flux.
Various expedients have been used to minimize contamination in the fusion zone during these welding processes. For example, some of the more obvious steps include mechanical cleaning of the metals to be joined by such techniques as scrubbing, grit blasting or chemical cleaning. In addition, the workpiece to be joined is preheated in order to drive off any adsorbed gases and moisture which may be present. Similarly, through selective care and handling of the filler metal rods, the impurities which could otherwise enter into the weld zone from this source are minimized. During the development of arc welding techniques, various gas shielding devices and techniques have evolved and they contribute significantly to the reduction of impurities in the weld zone. Additionally, the cover gas or flux as the case may be, can be maintained relatively contamination free subject to arc stability requirements. Another technique for minimizing the impurity content in the final weld is to use a filler rod having a suitable offsetting composition which complements the composition of the base metal in a manner which results in a final weld material composition that is within some preselected range.